The present invention relates to a dynamic pressure bearing device applicable to apparatus rotating at high speed, for example hard disk drivers, rotary polygon mirror drivers and so forth and methods of manufacturing such a device, and more particularly to a thrust dynamic pressure bearing device and a method of manufacturing the same.
Among apparatus rotating at high speed such as hard disk drivers, there is one using a dynamic pressure bearing so arranged that rotors can rotate without contacting bearing members by causing dynamic pressure to be generated in lubricating oil lying between the rotors and the bearing members while the rotors are rotating upon the respective bearing members. Such a dynamic pressure bearing includes a radial dynamic pressure bearing for supporting the rotor in the direction in which it rotates and a thrust bearing for supporting the rotor in the axial direction.
As for a known thrust bearing, it is formed by press-fitting a disk-like thrust plate into a rotary shaft in order to secure the thrust plate thereto.
The dynamic pressure bearing device will be briefly described by the use of an example of a hard disk driving motor shown in FIG. 6. The structure of the thrust bearing shown in FIG. 6 conforms to what is based on the present invention and though the motor structure is well known, the following description will be given to make clear problems to be solved by the invention.
In FIG. 6, a motor frame 8 has a hole for fixing a sleeve 4 and the lower end portion of the sleeve 4 is securely press-fitted into the hole. The sleeve 4 is a cylindrical member having a central shaft hole, and a small-diameter and a large-diameter recessed portions 13 and 14 are formed round the central shaft hole in the lower end portion of the sleeve 4. The central hole of a stator core 12 is placed in the outer periphery of the sleeve 4 and fixedly bonded thereto or fixed by a proper member. The stator core 12 has a proper number of radially projected poles and a driving coil 23 is wound on each projected pole.
A rotary shaft 1 is inserted into the central hole of the sleeve 4. A disk-like thrust plate 5 has been secured to the lower end portion of the rotary shaft 1 by press-fitting. The thrust plate 5 is disposed in the small-diameter recessed portion 13 of the sleeve 4. A counter plate 6 forming a fixed-side dynamic pressure bearing part is fixedly fitted in the large-diameter recessed portion 14 so as to positioned the thrust plate 5 in the small-diameter recessed portion 13.
Herringbone dynamic pressure grooves are formed on upper and lower surfaces 51 and 52 of the thrust plate 5 in the circumferential direction. There are very small gaps respectively between the upper surface 51 of the thrust plate 5 and the opposed sleeve 4 and between the lower surface 52 of the thrust plate 5 and the opposed counter plate 6. Lubricating fluid such as lubricating oil lies in these gaps, thus forming a dynamic pressure thrust bearing. When the thrust plate 5 together with the rotary shaft 1 rotates, the lubricating fluid is compressed in the dynamic pressure grooves, so that the dynamic pressure is generated in the thrusting direction. Moreover, a very small gap is also present between the outer periphery of the rotary shaft 1 and the central shaft hole of the sleeve 4, dynamic pressure grooves are formed in at least one of the outer peripheral face of the rotary shaft 1 and the inner peripheral face of the central shaft hole of the sleeve 4. Consequently, a radial dynamic pressure bearing part 40 is formed with lubricating fluid lying between the gap above. As the rotary shaft 1 rotates, the lubricating fluid causes dynamic pressure is generated in the radial direction.
The rotary shaft 1 projects from the upper edge face of the sleeve 4, and the central hole of a rotor hub 2 like a cup placed upside down is secured by press-fitting to the projected portion of the rotary shaft 1. The outer peripheral wall of the rotor hub 2 covers the stator core 12, and a cylindrical rotor magnet 7 is secured to the inner periphery of the outer peripheral wall. The central hole of a one or a plurality of hard disks (not shown) is placed with the outer peripheral face of the rotor hub 2 as a guide, and the hard disk(s) is mounted on a flange 21 formed on the outer peripheral face of the rotor hub 2. The hard disk is fitted integrally to the rotor hub 2 with a proper cramp member.
Supply of power to each driving coil 23 is controlled in accordance with the rotational position of the rotor magnet 7, whereby the rotor magnet 7, the rotor hub 1, the rotary shaft 1 and the thrust plate 5, these integrally forming rotating parts, are rotated. With their rotation, the aforementioned thrust and radial dynamic pressures are generated and the rotary shaft 1 makes non-contact rotation relative to the sleeve 4 and the counter plate 6 forming the fixed-side dynamic pressure bearing part. Therefore, the frictional resistance is reduced to make possible the smooth and high-speed rotation of the rotary shaft 1.
With respect to the formation of the thrust dynamic pressure bearing part, the rotary shaft 1 is press-fitted into and integrated with the thrust plate 5, for example. FIG. 7 shows how the rotary shaft 1, the thrust plate 5 and the counter plate 6 are combined together in the related art. As seen from FIG. 6, the right angles at which the rotary shaft 1 meets the thrust plate 5 are important and unless the degree of the right angle is satisfactory as shown in FIG. 8, the gap in the thrust dynamic pressure bearing part would lack uniformity, thus making the generation of the dynamic pressure unstable. The method heretofore used to increase the degree of the right angle is to raise the precision of jigs when the rotary shaft 1 is press-fitted into the thrust plate 5.
The degree of the right angle between the rotary shaft and the thrust plate may be considered to be made accurately achievable by shaving the shaft without press-fitting the rotary shaft into the thrust plate. Under this method, the degree of the right angle can be attained precisely and there is no problem arising from causing the thrust plate from warping. However, the disadvantage of the shaving method includes making the working troublesome, requiring a lengthy working time, wasting much raw material and increasing costs.
These problems will be developed from not only the shaft rotating type but also a shaft fixed type.
An object of the present invention made to solve the foregoing problems concerning the related art is to provide a dynamic pressure bearing device having a shaft and a thrust plate forming a thrust dynamic pressure bearing part which is mounted to the shaft in a direction perpendicular to the shaft, in such a manner as to integrate the shaft and the thrust plate by press-fitting so that the thrust plate can be set free from warping and the gap between the thrust plate and the fixed-side dynamic pressure bearing part is uniformized, whereby a stable thrust dynamic pressure is obtainable, and to provide a method of manufacturing the same.
In order to achieve the above object, according to the present invention, there is provided a dynamic pressure bearing device, comprising:
a shaft member;
a thrust plate, formed with a press-fitting portion into which the shaft member is press-fitted such that the thrust plate extends perpendicular to an axial direction of the shaft member;
a bearing member, being opposed to the thrust plate for forming a thrust dynamic pressure bearing portion; and
at least two relief portions, for absorbing press-fitting stress, the relief portions provided in at least one of the press-fitting portion of the thrust plate and a part of the shaft member which corresponds to the press-fitting portion.
In this configuration, a portion which is equivalent to the press-fitting margin is relieved when the shaft member is press-fitted into the thrust plate. Therefore, the stress axially applied to the thrust plate after it has been press-fitted into the shaft is reduced without causing the thrust plate to warp. Thus, the gap between the thrust plate and the opposed bearing member is uniformized and the shaft member after the generation of the dynamic pressure is determined with stability, so that a highly reliable dynamic pressure bearing device becomes obtainable.
Preferably, the relief portions are formed with equal intervals in a circumferential direction of the shaft member.
In this configuration, the press-fitting margin portion can be relieved with a well balance, which resulting in obtaining a stable dynamic pressure with the warping of the thrust plate effectively reduced.
More preferably, the relief portions are provided as notched grooves which extend entirely in the thrust plate, in parallel with the axial direction of the shaft member.
Preferably, the thrust plate is made of a material which can be subjected to coining process, and dynamic pressure grooves are formed on the thrust plate by the coining process.
Although such a material may be easily deformable, that is, easily warped at the time of press-fitting, the warping of the thrust plate can be efficiently eliminated by forming each relief portion for absorbing the press-fitting stress.
In order to manufacture the dynamic pressure bearing device, there are provided a shaft member, and a thrust plate formed with a press-fitting portion into which the shaft member is press-fitted. At least two relief portions are formed in at least one of the press-fitting portion of the thrust plate and a part of the shaft member which corresponds to the press-fitting portion. The shaft member is press-fitted with the press-fitting portion such that the thrust plate extends perpendicular to an axial direction of the shaft member, while making the relief portions absorb press-fitting stress.
Preferably, the relief portions are formed when the coining process is conducted to form the dynamic pressure grooves on the thrust plate.
In this configuration, the relief portions can be formed without increasing the number of processing steps.